An inner fin having a plurality of circular holes and being positioned within a tube for promoting the heat exchanging efficiency has been known, such as described in the Japanese laid open utility model publication 60-176379, Japanese utility model publication 60-21669.
However, since the conventional type of the inner fin also has a continuous wall on which no hole is formed, and since such the continuous wall is positioned along with a flow direction, a boundary layer is generated and grown along with the surface, as shown in FIG. 9. Such boundary layer should reduce the heat transfer efficiency (Nu). In other words, the inner fin having a circular holes cannot improve heat exchanging efficiently very effectively.